filedesc: simplify fget_unlocked & friends

Introduce fget_fcntl which performs appropriate checks when needed.
This removes a branch from fget_unlocked.

Introduce fget_mmap dealing with cap_rights_to_vmprot conversion.
This removes a branch from _fget.

Modify fget_unlocked to pass sequence counter to interested callers so
that they can perform their own checks and make sure the result was
otained from stable & current state.

Reviewed by: silence on -hackers